Re: [PATCH 2/2] vmsplice: Add limited zero copy to vmsplice

[Robert Jennings <rcj@linux.vnet.ibm.com>]

* Vlastimil Babka (vbabka@suse.cz) wrote:
> On 10/07/2013 10:21 PM, Robert C Jennings wrote:
> > From: Matt Helsley <matt.helsley@gmail.com>
> > 
> > It is sometimes useful to move anonymous pages over a pipe rather than
> > save/swap them. Check the SPLICE_F_GIFT and SPLICE_F_MOVE flags to see
> > if userspace would like to move such pages. This differs from plain
> > SPLICE_F_GIFT in that the memory written to the pipe will no longer
> > have the same contents as the original -- it effectively faults in new,
> > empty anonymous pages.
> > 
> > On the read side the page written to the pipe will be copied unless
> > SPLICE_F_MOVE is used. Otherwise copying will be performed and the page
> > will be reclaimed. Note that so long as there is a mapping to the page
> > copies will be done instead because rmap will have upped the map count for
> > each anonymous mapping; this can happen do to fork(), for example. This
> > is necessary because moving the page will usually change the anonymous
> > page's nonlinear index and that can only be done if it's unmapped.
> 
> You might want to update comments of vmsplice_to_user() and
> SYSCALL_DEFINE4(vmsplice) as they both explain how it's done only via
> copying.
> 
> Vlastimil
> 

I will update those as well.  Thanks.

> > Signed-off-by: Matt Helsley <matt.helsley@gmail.com>
> > Signed-off-by: Robert C Jennings <rcj@linux.vnet.ibm.com>
> > ---
> >  fs/splice.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 63 insertions(+)
> > 
> > diff --git a/fs/splice.c b/fs/splice.c
> > index a62d61e..9d2ed128 100644
> > --- a/fs/splice.c
> > +++ b/fs/splice.c
> > @@ -32,6 +32,10 @@
> >  #include <linux/gfp.h>
> >  #include <linux/socket.h>
> >  #include <linux/compat.h>
> > +#include <linux/page-flags.h>
> > +#include <linux/hugetlb.h>
> > +#include <linux/ksm.h>
> > +#include <linux/swapops.h>
> >  #include "internal.h"
> >  
> >  /*
> > @@ -1562,6 +1566,65 @@ static int pipe_to_user(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
> >  	char *src;
> >  	int ret;
> >  
> > +	if (!buf->offset && (buf->len == PAGE_SIZE) &&
> > +	    (buf->flags & PIPE_BUF_FLAG_GIFT) && (sd->flags & SPLICE_F_MOVE)) {
> > +		struct page *page = buf->page;
> > +		struct mm_struct *mm;
> > +		struct vm_area_struct *vma;
> > +		spinlock_t *ptl;
> > +		pte_t *ptep, pte;
> > +		unsigned long useraddr;
> > +
> > +		if (!PageAnon(page))
> > +			goto copy;
> > +		if (PageCompound(page))
> > +			goto copy;
> > +		if (PageHuge(page) || PageTransHuge(page))
> > +			goto copy;
> > +		if (page_mapped(page))
> > +			goto copy;
> > +		useraddr = (unsigned long)sd->u.userptr;
> > +		mm = current->mm;
> > +
> > +		ret = -EAGAIN;
> > +		down_read(&mm->mmap_sem);
> > +		vma = find_vma_intersection(mm, useraddr, useraddr + PAGE_SIZE);
> > +		if (IS_ERR_OR_NULL(vma))
> > +			goto up_copy;
> > +		if (!vma->anon_vma) {
> > +			ret = anon_vma_prepare(vma);
> > +			if (ret)
> > +				goto up_copy;
> > +		}
> > +		zap_page_range(vma, useraddr, PAGE_SIZE, NULL);
> > +		ret = lock_page_killable(page);
> > +		if (ret)
> > +			goto up_copy;
> > +		ptep = get_locked_pte(mm, useraddr, &ptl);
> > +		if (!ptep)
> > +			goto unlock_up_copy;
> > +		pte = *ptep;
> > +		if (pte_present(pte))
> > +			goto unlock_up_copy;
> > +		get_page(page);
> > +		page_add_anon_rmap(page, vma, useraddr);
> > +		pte = mk_pte(page, vma->vm_page_prot);
> > +		set_pte_at(mm, useraddr, ptep, pte);
> > +		update_mmu_cache(vma, useraddr, ptep);
> > +		pte_unmap_unlock(ptep, ptl);
> > +		ret = 0;
> > +unlock_up_copy:
> > +		unlock_page(page);
> > +up_copy:
> > +		up_read(&mm->mmap_sem);
> > +		if (!ret) {
> > +			ret = sd->len;
> > +			goto out;
> > +		}
> > +		/* else ret < 0 and we should fallback to copying */
> > +		VM_BUG_ON(ret > 0);
> > +	}
> > +copy:
> >  	/*
> >  	 * See if we can use the atomic maps, by prefaulting in the
> >  	 * pages and doing an atomic copy
> > 
> 

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